In the past, many different methods of supplying a regulated voltage have been used. One well known method is to use the zener breakdown voltage of a PN junction as the reference; however, this method is limited in accuracy to its positive temperature coefficient. A second method depends upon the MOSFET threshold voltage, V.sub.t, which is limited in accuracy to both the current flowing in the device as well as the variation of a number of process parameters.
A more accurate reference voltage may be achieved by using bandgap voltage reference circuits which are capable of providing output voltage levels below V.sub.DD that are stable over temperature and process variations. In the past, this has been accomplished using both bipolar and MOS circuits. U.S. Pat. No. 3,887,863 by Brokaw discloses a circuit of the former type using two bipolar transistors to generate the bandgap voltage and an operational amplifier which provides feedback and a resulting nearly zero temperature coefficient.
It is common in MOS bandgap voltage reference circuits to build substrate bipolar transistors which are used as emitter followers wherein the emitter of one transistor is much larger than the emitter of the second transistor. The same bias voltage is applied to the bases of both transistors causing different current densities to flow through the emitters which in turn generates a difference between the base-emitter voltages of the transistors (delta V.sub.be). The temperature stable reference is derived from the sum of V.sub.be (from the transistor having the larger emitter) and delta V.sub.be which are chosen to equal the bandgap voltage of 1.205 volts. This reference voltage is coupled to inputs of an operational amplifier whose output provides both feedback to the bases of the transistors as well as the reference voltage.
It is well known that the accuracy of MOS operational amplifiers depend on the offset voltage, typically 20 millivolts, which in turn reduces the accuracy of the output reference voltage below that which is achievable in the bipolar implementation. The output reference voltage is thus prone to variations in the MOS threshold voltages. Furthermore, a significant amount of area on chip must be used for the inclusion of the operational amplifier.
Thus, what is needed is a MOS bandgap voltage reference circuit that provides a reference voltage stable over temperature and process variations.